In general, inkjet printers include at least one printhead that ejects drops of liquid ink onto an image receiving surface. A phase change inkjet printer employs phase change inks that are solid at ambient temperature, but transition to a liquid phase at an elevated temperature. The melted ink can then be ejected onto an image receiving surface by a printhead. The image receiving surface may be a media substrate or an intermediate imaging member. The image on the intermediate imaging member is later transferred to an image receiving substrate. Once the ejected ink is on the image receiving surface, the ink droplets quickly solidify to form an image.
Printers store a variety of fluids to enable ink imaging and ensure reliable printer operation. In some cases, monitoring of the volume or the head height of the stored fluids is important. Accurate monitoring of the head height is especially important where the head height of a stored fluid affects the mechanism or system that draws or uses the fluid. For example, restricting the head height range within an ink reservoir and precisely controlling the replenishment to an on-board ink reservoir of a printhead are often needed to prevent overfill-caused dripping of ink from the printhead jet orifices and to prevent the introduction of air if the fluid level is depleted below tolerable levels.
Currently available fluid sensing systems suffer from a number of drawbacks. For instance, applications in which small reservoirs or holding tanks are needed to store a fluid may not offer the space or fluid height required to accommodate known fluid sensing systems, such as float-based systems. Also, many “sense and fill” systems suffer from significant hysteresis problems in that these systems tend to respond late or overfill before flow is stopped. Moreover, fluid sensing systems that sense fluid materials by detecting a resistance change upon attaining a liquid level are dependent on consistent material properties, which may change over the life of the mechanism or system that uses the fluid. For example, the properties of a fluid may deteriorate over time due to age degradation, or the fluid may be replaced with a fluid having different properties. Therefore, improvements to sensing systems that enable fluid sensing in small and irregular shape reservoirs and that can detect fluids with varying properties are desired. Improvements to sensing systems that can respond to small, short term fluid level changes and longer term, continuous changes where the initial fluid level may be at any point in the usable volume range are also desirable.